Columbia Seat tube diameter ?

[Superman1984]

Is it 5/8 at the top or where it slips into the frame?

Probably 5/8" at the top where the seat clamps. I know both my Columbias & some of my Space type Murrays use a very small seat clamp on the rails. Really Annoying because it's abnormally small when you're used to more modern stuff

 

[bloo]

Yeah, most vintage seat guts are 5/8", newer ones 7/8". Theoretically you should have a seat post to match your seat, but in 21.15 it would be a miracle to find a post at all.

 

[Ernbar]

No it’s 5/8 the entire piece